This invention relates in general to electrostatography and, more specifically, to a structurally simplified electrophotographic imaging member and processes for preparing and using the imaging member.
A photoconductive layer for use in electrophotography or xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer with the photoconductive layer capable of photogenerating holes and injecting photogenerated holes sandwiched between the contiguous charge transport layer and the supporting conductive layer, the outer surface of the charge transport layer is normally charged with a uniform charge of a negative polarity and the supporting electrode is utilized as an anode. Obviously, the supporting electrode may also function as an anode when the charge transport layer is sandwiched between the supporting electrode and a photoconductive layer which is capable of photogenerating electrons and injecting the photogenerated electrons into the charge transport layer. The charge transport layer in this embodiment, of course, must be capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer.
Various combinations of materials for charge generating layers and charge transport layers have been investigated. For example, the photosensitive member described in U.S. Pat. No. 4,265,990 utilizes a charge generating layer in contiguous contact with a charge transport layer comprising a polycarbonate resin and one or more of certain diamine compound. Various generating layers comprising photoconductive layers exhibiting the capability of photogeneration of holes and injection of the holes into a charge transport layer have also been investigated. Typical inorganic photoconductive materials utilized in the generating layer include amorphous selenium, trigonal selenium, and selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium-arsenic, and mixtures thereof. Typical organic photoconductive materials include benzimidazole perylenes, phthalocyanines, azo pigments, and the like. The charge generation layer may comprise a homogeneous photoconductive material or particulate photoconductive material dispersed in a binder. Other examples of homogeneous and binder charge generation layer are disclosed in U.S. Pat. No. 4,265,990. Additional examples of binder materials such as poly(hydroxyether) resins are taught in U.S. Pat. No. 4,439,507. The disclosures of U.S. Pat. No. 4,439,507 and U.S. Pat. No. 4,265,990 are incorporated herein in their entirety. Photosensitive members having at least two electrically operative layers as disclosed above provide excellent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely developed electroscopic marking particles. However, when the supporting conductive substrate comprises a metal having an outer oxide surface such as aluminum oxide, difficulties have been encountered with these photosensitive members under extended electrostatographic cycling conditions found in high volume, high speed copiers, duplicators and printers. For example, it has been found that when certain charge generation layers comprising a resin and a particulate photoconductor are adjacent an aluminum oxide layer of an aluminum electrode, the phenomenon of "cycling-up" is encountered. Cycling-up is the build-up of residual potential through repeated electrophotographic cycling. Build-up of residual potential can gradually increase under extended cycling conditions to as high, for example, as 300 volts. Residual potential causes the surface voltage to increase accordingly. Build-up of residual potential and surface voltage causes ghosting, increased background on final copies and cannot be tolerated in precision high-speed, high-volume copiers, duplicators, and printers.
It has also been found that photosensitive members having a homogeneous generator layer such as As2Se3 such as those disclosed in U.S. Pat. No. 4,265,990, exhibit "cycling-down" of surface voltage when operated at extended cycling conditions found in high speed, high volume copiers, duplicators and printers. When cycling-down occurs the surface voltage and charge acceptance decrease as the dark decay increases in the areas exposed and the contrast potential for good images degrades and causes faded images. This is an undesirable fatigue-like problem and is unacceptable for high speed, high speed, high volume applications.
U.S. Pat. No. 4,464,450 discloses the fabrication of an electrophotographic imaging member having two electrically operative layers including a charge transport layer and a charge generating layer which overlie a siloxane film coated on a conductive metal/polymer film supporting substrate. The siloxane film is a cross-linked coating comprising a reaction product of a hydrolyzed silane having an amine functional group to eliminate hole injection from the underlying conductive metal/polymer film substrate and suppress cycle-down problems of a system utilizing negative charging electrophotographic imaging processes. Electrophotographic imaging members fabricated with this silane hole blocking layer described above to provide electrical cyclic stability have produced excellent electrophotographic imaging members. However, because the hydrolysis process for the silane and the intermolecular crosslinking reaction to form the siloxane coating are spontaneous, it can be difficult to consistently control the quality of the outcome. The formation of islands of siloxane aggregates and thickness nonuniformity may occur in some silane coating. The siloxane aggregates have been identified as a possible source associated with the white spot printout. Also silane coating thickness nonuniformity is seen to adversely affect copy quality. Moreover, the silane's inherent hydrophilic characteristics can weaken the adhesion bond strength at the silane/metal interface. Hence, silane layer delamination may occur when the imaging belt is cycled in a machine in a high humidity environment. Moreover, a separate adhesive layer is often needed to provide good adhesion linkage between the silane layer and the charge generating layer. When fabricated into an imaging belt by ultrasonic seam welding techniques, seam delamination/cracking may occur as the belt flexes over small diameter (e.g., 19 ram) belt support rollers under humid conditions. Since the silane hole blocking layer is applied to the electrophotographic imaging member as a separate coating step, it can reduce imaging member fabrication throughput by up to 33 percent and increases material and manpower costs. Each extra coating step, such as the silane coating step, also reduces production yields by increasing the likelihood of damage due to scratches caused by handling and coating defects.
In the fabrication of seamless flexible electrophotographic imaging members employing a conductive polymeric support substrate, i.e. a substrate that does not utilize an inorganic conductive layer, silane coatings are not acceptable as a hole blocking layer because the substrate support has no metal oxides with which the silanol groups of the silane layer can react to form adhesive bonds.
For some inverted electrophotographic imaging member designs using a positive electrostatic charging process (e.g., where the charge transport layer is sandwiched between a conductive substrate and a charge generating layer) the silane layer on the exposed outer surface of the charge generation layer of the imaging member tends to wear rapidly and requires an overcoating iayer for protection.
The formation and development of electrostatic latent images on the imaging surfaces of photoconductive materials by electrostatic means is well known as disclosed, for example, in U.S. Pat. No. 4,265,990, U.S. Pat. No. 4,618,551, and U.S. Pat. No. 4,346,158. The disclosures of the aforementioned patents are incorporated herein by reference in their entirety.